[ceph-users] Why keep old epochs?

Gregory Farnum gfarnum at redhat.com
Sun Nov 19 01:45:24 PST 2017

On Wed, Nov 15, 2017 at 2:07 PM Bryan Henderson <bryanh at giraffe-data.com>

> Some questions about maps and epochs:
> I see that I can control the minimum number of osdmap epochs to keep with
> "mon min osdmap epoch".  Why do I care?  Why would I want any but the
> current
> osdmap, and why would the system keep more than my minimum?
> Similarly, "mon max pgmap epoch" controls the _maximum_ number of pgmap
> epochs
> to keep around.  I believe I need more than the most recent pgmap because I
> need to keep previous ones until all PGs that were placed according to that
> pgmap have migrated to where the current pgmap says they should be.  But
> do I
> need more epochs than that, and what happens if the maximum I set is too
> low
> to cover those necessesary old pgmaps?

Ah, you're a bit backwards here. Despite, the name, the "pgmap" is a
reporting structure about how much space PGs use, but it *does not* control
where the PGs are placed. That's entirely determined by the OSDMap.

So the cluster needs older OSDMaps to do peering and make sure it gets
up-to-date PG data when doing replication/peering/recovery/etc. The PGMaps
are only useful for reporting statistics.

I don't think you as a user have any good reason to mess around with these
at this point.

> --
> Bryan Henderson                                   San Jose, California
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